HR: 1330h
AN: H52A-1143    [PDF]
TI: The Origin of Convex Waterfalls Along the Niobrara River by Valentine, Nebraska
AU: * Pederson, D T
EM: dpederson2@unl.edu
AF: Univ. of Nebraska-Lincoln, 214 Bessey Hall Dept. of Geosciences, Lincoln, NE 68588-0340 United States
AU: Mason, L J
EM: lmason2@bigred.unl.edu
AF: Univ. of Nebraska-Lincoln, 214 Bessey Hall Dept. of Geosciences, Lincoln, NE 68588-0340 United States
AU: Goble, R J
EM: rgoble2@unl.edu
AF: Univ. of Nebraska-Lincoln, 214 Bessey Hall Dept. of Geosciences, Lincoln, NE 68588-0340 United States
AB: Waterfalls on tributaries along a stretch of the Niobrara River east of Valentine, Nebraska have a convex-downstream geometry in the horizontal plane and an arc in the vertical plane. Siltstone forming the cliff face is only slightly indurated and should be easily eroded by stream water. This is not the case as the cliff face on either side of the waterfalls clearly undergoes more rapid erosion than the face of the falls forming a convex geometry. This differential erosion rate includes points where waterfalls occur immediately along the banks of the Niobrara River. At these points, riverbank erosion is clearly more effective upstream and downstream of a waterfall as evidenced by the protrusion of the waterfall face into the river. There appears to be repetitive-cycles of waterfall evolution along the tributaries. The convexity increases until collapse occurs on the face of the falls, headward erosion then occurs, followed by the development of a new convex waterfall. The waterfalls have a more typical geometry (convex upstream) on tributaries in the lower reaches of the Niobrara River, where cliff faces become coarser grained. The tributaries are fed by springs. Several processes interact to form the convex geometry. Freeze/thaw erosion is more effective on either side of the waterfalls. Water enters the face of the falls and wicks into the cliff causing negative pore pressures. Clay coatings on mineral grains appear to play a role in protection from erosion. Algae and lichen growth aids in protecting the face of the falls. Small groundwater flow systems, represented by seepage into the streambed above the falls and discharge to the side of the falls can cause seepage erosion and enhancement of freeze/thaw erosion.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1829 Groundwater hydrology
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting